单项选择。

The poor little boy often stands _____ the gate. He is waiting for his mother. [ ]

A. in front of

B. in front the of

C. in the front of

D. front of

用皮碗式密封的滚动轴承多用在（）的场合。

A、高温

B、高速

C、外露

D、环境清洁